Virtual endoscopy is a non-invasive diagnostic procedure aimed at exploring the inner surface of anatomical structures inside the human body. Using advanced image-processing techniques, three dimensional (3D) models are reconstructed from a series of high-resolution two dimensional (2D) images. A physician can then navigate inside the 3D virtual model either manually or using an automatic navigation algorithm.
One of the most promising uses for virtual endoscopy is the screening of patients for colorectal cancer. Virtual endoscopy mimics conventional colonoscopy with the added advantage that it is less invasive and potentially more attractive as a screening method. An additional benefit of virtual colonoscopy over conventional colonoscopy is the ability to fly-through the colon in both an antegrade and retrograde direction. Although this dual directional navigation significantly increases the amount of surface displayed, substantial sections of the colon remain obstructed behind haustral folds.
A number of techniques have been proposed to create image projections that overcome some of the limitations of the standard 3D visualization technique. These techniques seek to display more of the surface of the colon and thereby increase the polyp detection sensitivity. Some of the views are generated by placing the virtual camera in the center of a viewing space, and projecting images onto the corresponding walls.
One known technique uses a cubic viewing space with the cameras located in the center of the cube and projecting on each face an image with a 90 degree viewing angle. The cube is then unfolded into a single plane presenting a 360 degree field of view of the colon surface. An example of this projection is shown in FIG. 1. To reduce the disturbance due to the discontinuities that arise with this representation, small images are added as flaps adjacent to each face. Off-line animated image sequences are generated from a number of point samples selected along the central path through the colon. FIG. 1 shows a single frame of an animated sequence using this cubing mapping. Each square shown in FIG. 1 represents a different side of the cube. As shown, the sides are labeled as left 102, bottom 104, front 106, top 108, right 110 and back 112. The unfolded cube forms a cross-like structure. While this technique shows a projection of multiple views in the same frame, the construction of the layout is difficult to follow and has black areas which disrupt the viewing area. As a result, a polyp that appears near an edge of the cube can be split into two or more windows when the cube is unfolded.
Another known technique uses map projections that visualize the entire surface of a viewing sphere. Using Mercator and stereographic projections, the surface of the sphere is transformed onto a flat surface. The major drawback of this technique is that, as any projection of the surface of a sphere on a plane, it introduces some degree of deformation of the image. In particular, the Mercator projection maps the poles of the globe infinitely far away while displaying the objects near the equator with minimum distortion. There is a need for a method for creating a panoramic endoscopic view from a volumetric image that efficiently displays the views and is minimally distorted.